Diffraction gratings and more complex thin holograms, encoded onto programmable liquid crystal (LC)-based spatial light modulators (SLMs), have been actively researched as a way to alter the wavefront of an optical beam. For example, these LC/SLMs may be used for adaptive-optic phase correction, in a synthetic phase array, or in a telecommunication beam steering switch. The LC/SLMs are based typically on either a transmissive or reflective type micro-display panel in order to provide the small pixel pitch requirement. LCs with both in-plane (e.g., such as in-plane-switching (IPS) using nematic LC and ferroelectric LC) and out-of-plane (e.g., planar or parallel aligned (PA) and vertical aligned (VA) nematic LC) rotation of LC director are utilized. The ferroelectric LC (FLC) will be polarization insensitive if the hologram is configured with two phase levels. Polarization insensitivity can be important for systems where the light source has unknown or scrambled polarization, such as for a beam-steering switch used in telecommunication networks. On the other hand, since out-of-plane switching nematic LCs (e.g., PA and VA nematic LC) are known to be polarization sensitive, holograms recorded onto these LC/SLMs generally require a known polarization. Accordingly, these types of LC holograms are typically only useful in optical systems and instrumentation where the light sources are polarized.
Although programmable thin holograms encoded onto LC/SLMs are very versatile, these active components are not cost effective for many applications. In addition, these programmable thin holograms are known to provide relatively small steering angles. For example, a state-of-the art LC on Silicon (LCoS) panel may have less than 10 μm pixel pitch, which at a wavelength of 0.5 μm and utilizing a minimum of two pixels per grating period, provides a maximum beam deflection angle of about 1.4 degrees. All other programmable hologram output (e.g., termed the replay) will have even smaller deflection angles.
Nevertheless, there has been interest in forming passive diffraction gratings or holograms based on these active device. For example, in U.S. Pat. No. 6,304,312, a diffraction grating is formed by injecting liquid crystal monomer between two transparent substrates, each of which is coated with an alignment layer. In one example, the alignment layer is uniform and the diffraction grating is effected by applying a voltage to patterned electrodes provided on the transparent substrates. In another example, the diffraction grating is effected with a patterned alignment layer (e.g. patterned using a photolithography technique). After the liquid crystal layers are aligned, they are then polymerized and/or cross-linked to fix the alignment. Note that the liquid crystal polymer pixels in this reference are limited to having either homeotropic alignment (i.e., perpendicular to the substrate) or planar alignment (i.e., parallel to the substrate). The resulting binary grating (e.g., having a pitch of about 8 μm) is reported to provide only about forty percent diffraction efficiency.
More recently, patterned photo-alignment layers having an even smaller pixel pitch (e.g., 1 μm or shorter) have been proposed. For example, in U.S. Pat. No. 7,375,784 a micro-patterned alignment layer is disclosed. While the alignment layer is limited to having only homeotropic alignment (i.e., perpendicular to the substrate) and planar alignment (i.e., parallel to the substrate), the liquid crystal may be aligned with a range of out-of-plane angles. More specifically, local alignment of the liquid crystal is stated to be determined by the average areas of underlying homeotropic alignment and planar alignment regions. Unfortunately, since the alignment of the liquid crystal is related to an average of different regions it cannot be patterned with precision and thus, is not suitable for many applications.
In fact, in order to optimize precision and cost effectiveness, most applications requiring passive holograms use diffractive optical elements with physical steps. Unfortunately, the etching and/or molding processes used to form these diffractive optical elements are relatively complex and time consuming. In addition, the surface relief structure generally requires complex optical thin-film coating processes to protect the delicate structures.